Adequate and reliable braking for vehicles, particularly large tractor-trailer vehicles is desirable. While drum or disc wheel brakes are capable of absorbing a large amount of energy over a short period of time, the absorbed energy is transformed into heat in the braking mechanism.
Multi-cylinder internal combustion engines, particularly diesel engines for large tractor-trailer trucks, may include an exhaust-gas turbocharger. The turbocharger includes a turbine that drives a compressor via a shaft, which generates an increased intake air pressure in the intake duct during normal operation.
Braking systems are known which include exhaust brakes which inhibit the flow of exhaust gases through the exhaust system, and compression release systems wherein the energy required to compress the intake air during the compression stroke of the engine is dissipated by exhausting the compressed air through the exhaust system.
In order to achieve a high engine-braking action a brake valve in the exhaust line may be closed during braking, and excess pressure is built up in the exhaust line upstream of the brake valve. The built-up exhaust gas flows at high velocity into the turbine and acts on the turbine rotor, whereupon the driven compressor increases pressure in the air intake duct. The cylinders are subjected to an increased charging pressure. In the exhaust system, an excess pressure develops between the cylinder outlet and the brake valve and counteracts the discharge of the air compressed in the cylinder into the exhaust tract via the exhaust valves. During braking, the piston performs compression work against the high excess pressure in the exhaust tract, with the result that a strong braking action is achieved.
Another method disclosed in U.S. Pat. No. 4,395,884 includes employing a turbocharged engine equipped with a double entry turbine and a compression release engine retarder in combination with a diverter valve. During engine braking, the diverter valve directs the flow of air through one scroll of the divided volute of the turbine. When engine braking is employed, the turbine speed is maximized, and the inlet manifold pressure is also maximized, thereby maximizing braking horsepower developed by the engine.
Other methods employ a variable geometry turbocharger (VGT). When engine braking is commanded, the variable geometry turbocharger is “clamped down” which means the turbine vanes are closed and used to generate both high exhaust manifold pressure and high turbine speeds and high turbocharger speeds. Increasing the turbocharger speed in turn increases the engine airflow and available engine brake power. The method disclosed in U.S. Pat. No. 6,594,996 includes controlling the geometry of the turbocharger for engine braking as a function of engine speed and pressure (exhaust or intake, preferably exhaust). U.S. Pat. No. 6,148,793 describes a brake control for an engine having a variable geometry turbocharger which is controllable to vary intake manifold pressure. The engine is operable in a braking mode using a turbocharger geometry actuator for varying turbocharger geometry, and using an exhaust valve actuator for opening an exhaust valve of the engine.
Other methods of using turbochargers for engine braking are disclosed in U.S. Pat. Nos. 6,223,534 and 4,474,006.
Controlled engine exhaust gas recirculation is a known technique for reducing oxides of nitrogen in products of combustion that are exhausted from an internal combustion engine to atmosphere. A typical EGR system comprises an EGR valve that is controlled in accordance with engine operating conditions to regulate the amount of engine exhaust gas that is re-circulated from the engine exhaust system to the air intake system so as to limit the combustion temperature and hence reduce the formation of oxides of nitrogen during combustion. Such a system is described for example in U.S. Pat. No. 7,363,761.